1. Field of the Invention
This invention relates to electrical motor type power-steering systems and particularly to power-steering systems used in automotive vehicles.
2. Description of the Prior Art
Japanese published unexamined patent application No. 59-77966 discloses a prior art motor-assisted power-steering system. This system comprises an electrical motor with a drive gear, a gear reducer comprising a train of spur gears meshing with the drive gear, a pinion shaft constituting a lower part of a steering shaft assembly and having a final stage spur gear of the gear reducer, a rack meshing with the pinion of the pinion shaft and pivoting road wheels. The motor rotates the pinion shaft and boosts the steering force exerted by the driver according to the angular deviation from the neutral position of a steering wheel fixed to an upper part of the steering shaft assembly. In this system, the dimensions of the gear reducer necessarily increase as the reduction gear ratio increases.
Japanese published unexamined patent application No. 58-141963 discloses another prior art electrical motor-assisted power-steering gear. This power-steering gear comprises an electrical motor with a cylindrical rotor and metal core, and a planetary gear reducer comprising a sun gear rotating together with the rotor and a ring gear rotating together with a steering shaft. The motor and the planetary gear reducer are fitted onto the steering shaft. The motor rotates the steering shaft and boosts the steering efforts of the driver. In this power-steering gear, the arrangement of a planetary gear means the gear reducer will be relatively complicated and the reduction gear ratio will be relatively low.